Blood cell development, or hematopoiesis, generates a variety of distinct cell lineages including lymphocytes, erythrocytes, megakaryocytes, monocytes, and granulocytes. These cells perform a diverse array of biological functions: they secrete antibodies, display cytotoxicity for tumor cells, transport molecular oxygen, synthesize and export growth factors, and destroy pathogens. Although the molecular mechanisms underlying hematopoietic development have not been completely resolved, it is clear that lineage-specific transcription factors are critical to this process. PU.1 (Spi-1) and Spi-B are highly related proteins that represent a subgroup of the Ets family of transcription factors. Ets factors have been studied extensively as key regulators of cellular differentiation and oncogenesis. By generating null mutations in mice, we have shown that both PU.1 and Spi-B are essential to hematopoiesis. PU.1 plays an unanticipated role in the proliferation, survival, and differentiation of multi- potential lymphoid-myeloid progenitors. This results in a complete loss of B cells, T cell macrophages, and neutrophils in PU.1-/- mutant mice. In direct contrast, Spi-B is dispensable for blood cell development but crucial for B lymphocyte expansion and antibody secretion upon stimulation of the B cell receptor (BCR) by antigen binding. In the studies described in this application, we will use a combination of biochemical and genetic approaches to precisely define their unique roles in blood cell development and/or function. Specifically, we will (i) determine why PU.1 is essential for proliferation and differentiation of lymphoid-myeloid progenitors, (ii) study how the transcriptional activity of PU.1 is regulated during lymphopoiesis and myelopoiesis, (iii) determine how Spi-B transcriptional activity is regulated in mature B cells, (iv) determine the extent to which PU.1 and Spi-B play redundant roles during hematopoiesis, and (v) characterize the BCR signaling defect that leads to apoptosis of PU.1+/- Spi-B+/- lymphocytes. These studies will have important implications for our understanding of how related transcription factors promoter different aspects of blood cell maturation.